Dict: support efficient key+value iteration

[ash2k]

I'd like to go over keys and values in a Dict, process them and put results somewhere else. I see two options:

• Iterate() Iterator allows me to go over keys. I can gen get the corresponding values using Get(Value). This is suboptimal because of the extra hashing+lookup costs.
• Items() []Tuple allows me to get all keys+values in one go. This is suboptimal because of the extra allocation to hold all the data.

I'd like a more efficient way that avoids the above issues.

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It's a breaking change to introduce a new method to Iterator to get the value of the mapping (it iterates over keys for Dict) so perhaps we can have a new interface (e.g. KeyValueIterator/MappingIterator) with one more extra method (or a method to get both values in one go)? And then perhaps one more interface to extend IterableMapping with a method to get the new kind of iterator (e.g. IterateKeysAndValues() KeyValueIterator or IterateMapping() MappingIterator).

keyIterator already has the value in the entry, it just lacks a way to get it.

[adonovan]

Go itself seems likely to add support for language-level iterators which will open up many exciting possibilities for the Starlark API. For example, it may be possible to say for k, v := range dict.All { ... } without the need for an explicit iterator, or a call to iter.Done, and without allocating an array proportional to dict.Len().

I think we should wait to see what happens there before committing to new API in Starlark.